The float process for the production of polished flat glass, is a method well known in the art. In practice, molten glass passes out of the melting furnace following which it is passed downstream into a heated float bath where it forms a continuous glass ribbon. In this chamber, the glass ribbon floats on the surface of a molten metal (e.g. tin) bath in a controlled atmosphere at a high enough temperature and for a sufficient period of time to even out any irregularities on the surface of the glass. Because the surface of the molten tin is flat, the surface of the glass also becomes flat.
Then the glass ribbon is removed from the bath and it gets outside from the float bath chamber by passing through airtight locks located downstream from the molten metal bath until the surfaces of the ribbon are hard enough for the glass to be removed from the bath without any marking of the bottom surface by the rollers.
In conjunction with the production of a glass ribbon on float baths of the type described above, French Pat. No. 2,542,636 discloses a method whereby a nozzle arrangement is utilized to spray metallic powders, which are then thermally degraded due to the heat of the glass ribbon, in order to form pyrolyzed coatings on the moving surface of the glass.
It is also known to utilize an aspirator within the zone where the powder is sprayed onto the glass in order to eliminate excess powder which does not adhere to the glass, as well as the waste products which result from the pyrolytic decomposition of the powder upon contact with the heated glass.
French Patent Application No. 85 00 134 discloses an aspirator which is associated with a blowing mechanism. This apparatus is configured and located so as to affect the atmospheric flow in the powder distribution zone. In operation, the use of this combination creates eddies in the powder proximal to the nozzle which does not immediately become affixed to the glass ribbon. This increases the opportunities for the powder to become deposited on the glass and reduces the chance that this unbound powder will be aspirated out of the chamber at the same time as the decomposition waste products, thus increasing the efficiency of the coating operation.
An installation of this type, however, functions well only with powders having a very precise particle size. If the powder particles are coarser than originally anticipated, or if initially suitable particles become agglomerated to form clusters, the powder may possess enough energy to travel out of the deposition zone to an area near the output of the float enclosure where the reducing gas which escapes the enclosure is burned away.
In the production of polished flat glass by the float bath procedure, the tin float bath is protected from oxidation by a protective atmosphere, preferably consisting of hydrogen-enriched nitrogen. To prevent the entry of air into the bath, this protective atmosphere is introduced at a slightly positive pressure into the enclosure.
Further, a series of airtight curtains, preferably at least three, bar the output of the float enclosure above the glass ribbon as it exits from the enclosure. As a result of the positive pressure exerted by the hydrogen-enriched nitrogen, a portion of the protective atmosphere escapes from the float enclosure between the curtains and the glass ribbon and underneath the ribbon. These gasses are then ignited beyond the airtight curtain located furtherest downstream due to the high temperature under which this operation is carried out.
This then, is the location of the burning gas, near which undesirably large or agglomerated coating particles may become deposited, as discussed above. Due to the heat of the flames at the output end of the float bath, however, the coarse particles or clusters of powder are consumed in a very short time, forming an unsightly blemish upon the glass and thus rendering it unmarketable.
One object of the present invention is to prevent the formation of blemishes upon the coated surface of the glass caused by the reaction of coarse powder particles or agglomerated clusters of these metal coating compounds upon contact with the flames at the output end of the float enclosure.
A further object of the invention is to reduce the constraints upon the size of the particles which may be chosen for deposition onto a glass ribbon by thermal decomposition.
One method previously contemplated for increasing the efficiency of the coating process is to partially raise the airtight curtain located at the output of the float bath, which delineates the outer boundary of a series of cooling locks. These cooling locks comprise a plurality of chambers filled with the same non-oxidizing atmosphere used in the float bath, wherein the glass ribbon is gradually allowed to cool to a degree which allows it to be removed from the enclosure without the formation of any imprint upon its bottom surface caused by the surface of the rollers. Raising this outer curtain, however, leads to the formation of flames, caused by the combustion of the escaping gas, at the base of a curtain further upstream which may disturb or even damage the operation of the float bath itself.
Devices have also been constructed which direct non-flammable gasses from the boundary of the particle deposition zone towards said deposition zone so as to form a barrier between the flames produced by the burning gas and the particles which land in that zone and to dilute the flammable gas. This arrangement interferes, however, with the mechanism of particle distribution in that it disturbs the eddy system discussed above in relation to French Patent Application No. 85 00 134.
Applicants have therefore discovered an improved apparatus and method for preventing the formation of blemishes in a metal oxide coating deposited upon a glass ribbon which is produced in an adjacent molten metal float bath. These blemishes are caused by the interaction of the powder particles with the flames located at the outlet end of the float bath. The use of applicants' novel apparatus and method will not disturb the deposition of the coating on the glass ribbon or the production of glass in the float bath.